In a printing operation, the term “registration” is the alignment of a print medium with respect to a printing element, such as a thermal printhead, in the in-line or print-medium-conveying direction (x axis direction) and in the lateral or direction orthogonal to the print-medium-conveying direction (y axis direction). The term “skew” is the rotation of the print medium with respect to the printing element. In other words, “registration” is caused by longitudinal and/or horizontal offsets whereas “skew” is caused by rotational offsets.
Proper registration and skew alignment of a print medium with respect to a printing element is of considerable importance, especially for economical, high-speed, high-volume print reproduction. For example, in a continuous web of print medium, such as a label or sheet of paper, the continuous web of print medium may be fed past printing subsystems that form images by applying one or more colorants to the print medium. From label to label, or from sheet to sheet, on a continuous web it is important for the image to be printed on the print medium in the same place on every label or sheet on the continuous web and in each case in the precise area of the label or sheet where the image is intended to be displayed. Even with respect to a single label or sheet, when printing an image on that label or sheet involves more than one color, depending on the method of printing, it is necessary to print the image one separate time for each separate color, and alignment of each image with respect to the others is imperative. Hence, there is a need for precise registration and skew alignment each time an image is applied to the same or different print medium in a printing operation.
For this reason, at the start of a printing operation, and at predetermined times throughout, an operator may ensure that the registration and skew of the print medium with respect to the printing element are properly aligned. In the course of high-speed, high-volume print reproduction processes however, the alignment of print medium to printing element may experience a degradation over time due to frictional slippage of the print medium as it is being conveyed through the printer by rollers or other conveyance mechanisms during a printing operation. Other factors may contribute to the degradation of the alignment. For example, heat generated by the printer, vibration of the mechanical components, wear and tear of the mechanical components, and even discontinuities in the quality along a web of the print medium, as well as other factors, may contribute to a drift of the registration and skew of the print medium with respect to the printing element. As a consequence of such drift, images may be printed on the print medium that are not in alignment with the printing element; a problem that is unacceptable, especially where precision printing is required, such as in connection with the printing of instruments and tickets. For tickets and certified documents, for example, the location of every graphical structure is important, such as for use in detecting counterfeit labels. Every misalignment is a hint that something is wrong.
To locate a print medium accurately with respect to a printing element, an operator, or in the case of an automated printer, the printer itself, needs to know two criteria—namely where the print starts and where the print is to be located. The edge of the label or sheet of paper often provides the first criterion and the point at which the image is to be applied to the label or sheet of paper is often used to provide the second criterion.
In conventional manual alignment practices in the printing industry, a grid of vertical and horizontal lines, typically spaced 0.1 mm apart, may be provided on a template label or sheet of paper. The intersection of each one of the vertical and horizontal lines forms a cross-hair. The template label or sheet of paper may be individually inserted into the print station for the purpose of performing a print registration and skew alignment test. Alternatively, on a web of labels or sheets of paper, the template label or sheet may be interspersed on the web in between labels or sheets of paper destined for production printing for the purpose of periodically performing a print registration and skew alignment test during a print operation, manually or dynamically, every time the template label or sheet reaches the print station. To perform the registration and skew test, the printer applies a dark mark, such as a cross-hair, onto the test label or sheet with the grid. In a manual operation, the operator may then measure the position of the dark mark with respect to a reference point on the grid using a special ruler with 0.1 mm scale and a magnifying glass. In an automatic operation, the printer itself may make the measurement. The displacement of the cross-hair from the reference point on the grid represents the directional drift of the print medium on the conveyor belt with respect to the printing element. Manual or automatic correction of this displacement sets the registration and skew back into alignment.
Manual measurement of print registration and skew drift using the foregoing technique requires a visual observation and interpretation of the cross-hair to the reference point on a 0.1 mm scale which is imprecise, subjective, and introduces inconsistency in registration and skew alignment, depending upon the person taking the measurement. Automatic measurements of print and skew drift using the foregoing technique, albeit not subject to the impreciseness and subjectivity of a visual observation used in a manual method, is nonetheless limited by the precision afforded by the grid used on the template label or sheet of paper.
There is a need for improved registration and skew alignment of print medium to printing element and this disclosure provides one such improvement.